The present invention relates to a method of preventing the breaking of a wire electrode such as may occur during the automatic operation of a wire electrode type electric discharge machining device. More particularly, the invention relates to a method of predicting the breaking of the wire electrode in a wire electrode type electric discharge machining operation by detecting preliminary symptoms of the breaking of the wire electrode.
An electrode type electric discharge machining device of the type to which the invention pertains is disclosed, for instance, by Japanese Published Patent Application No. 10856/1985. The general arrangement of the wire electrode feeding mechanism of this machining device is shown in FIG. 1.
In FIG. 1, reference numeral 1 designates a wire electrode about 0.2 mm in diameter made, for instance, of brass; 2, a wire supplying reel for supplying new wire electrode as the wire electrode 1 is consumed; 3, a wire electrode drive motor for pulling the wire electrode 1 upwardly at a certain speed; 4, a pinch roller used together with the roller of the drive motor 3 to guide the wire electrode; 5, a brake for stretching the wire electrode tight in the machining region A; and 6, a pinch roller used together with the roller of the brake 5 to hold the wire electrode 1 therebetween to transmit the braking force to the wire electrode.
Furthermore in FIG. 1, reference numeral 7 designates a wire electrode winding motor for winding wire electrode already used for machining and pulled upwardly by the drive motor 3; 8, a wire electrode winding reel; 10, a brake for stretching the wire electrode 1 tight in the region B; 11, a workpiece to be machined; 12, an electric current supplying contact member; and 13, a power supply unit for supplying a voltage across the workpiece 11 and the wire electrode 1. In the conventional device, the brake 5 and the pinch roller 6 are used to stretch the wire electrode tight in the machining area A.
A serious abnormal condition of the wire electrode type electric discharge machining device thus arranged is the breaking of the wire electrode, which usually occurs unexpectedly. When breaking of the wire electrode occurs, the machining operation must be suspended. In this case, an automatic wire electrode supplying device is operated to feed the wire electrode 1 through the upper and lower dies (not shown) so that the machining operation can be started again.
It is impossible to pass the wire electrode through the dies again at the position relative to the workpiece where the breakage of the wire electrode occurred; that is, it must be done at the initial hole (approach hole) on the workpiece, namely, the machining origin point. Thereafter, the wire electrode is moved along the original profile to the position where the breakage occurred.
When the wire electrode breaks, vertical stripes are formed in the surface of the workpiece at the position where the wire electrode broke. Therefore, it is essential to employ countermeasures to prevent breakage of the wire electrode as described above. To do this, it is essential to detect phenomena preliminary to the breaking of the wire electrode, thereby to prevent the breaking of the wire electrode. For this purpose, recently a method of statistically processing signals representative of the machining current or voltage or of machining sounds picked up through a microphone to detect abnormal conditions has been studied.
Conventionally, it has been the practice to detect phenomena preliminary to the breaking of the wire electrode merely by processing such signals without taking the actual wire electrode breaking mechanism into consideration. Therefore, the method is effective only in some cases. Further, the conventional method is effective to detect the phenomena preliminary to breaking only several seconds to several tens of seconds before the actual breaking of the wire electrode occurs because it takes time to process the signals. Accordingly, the method is not effective at all in cases where the breaking of the wire electrode occurs abruptly.